1. Field of the Invention
The present invention relates to a communication apparatus and a communication method, and more particularly to a communication apparatus that performs communication with a communication medium such as a card on which a non-contact integrated circuit (IC) chip is mounted or a mobile device, and a communication method.
2. Description of the Related Art
For example, a communication apparatus that functions as a reader and a writer is incorporated into an automatic ticket gate. The communication apparatus performs communication with a communication medium by supplying power to the communication medium when the communication medium approaches the communication apparatus.
FIG. 10 is a block diagram illustrating an example of the general configuration of a communication apparatus and a communication medium.
A communication apparatus 100 has a data processing unit 101 that processes data to be transmitted and received data, a modulation generating circuit 102 that generates a signal for modulation from the data to be transmitted, and a carrier wave generating circuit 103 that generates a carrier wave. The output of the modulation generating circuit 102 and the carrier wave generating circuit 103 is input to a modulator 104, and the carrier wave generated by the carrier wave generating circuit 103 is modulated using the signal generated by the modulation generating circuit 102. The modulated signal is amplified by a transmission amplifier 105 and output to an antenna 106. The antenna 106 is connected to a detection circuit 107 that detects the received signal, and the detected signal is demodulated by a demodulation circuit 108 and sent to the data processing unit 101.
On the other hand, a communication medium 110 has an antenna 111 that supplies power and that transmits or receives data by electromagnetically connecting to the antenna 106 of the communication apparatus 100 through electromagnetic induction. In addition, the communication medium 110 has a detection/demodulation circuit 112 that extracts data from a signal received by the antenna 111, a data processing unit 113 that processes the extracted data, and a modulation circuit 114 that modulates the processed response data and that sends the data to the antenna 111.
The communication apparatus 100 can perform non-contact communication with the communication medium 110 when the communication medium 110 approaches the communication apparatus 100. If the distance between the communication apparatus 100 and the communication medium 110 changes during the communication, the communication conditions also change and accordingly the communication quality may change. For example, when the communication medium 110 approaches the communication apparatus 100, a phenomenon in which the tuning frequency deviates may occur and accordingly the communication quality may deteriorate as the communication medium 110 approaches the communication apparatus 100. With regard to this phenomenon, a technology has been provided in which the tuning frequency does not change even if the distance between the communication apparatus 100 and the communication medium 110 changes, thereby stabilizing the communication quality (refer to, for example, Japanese Unexamined Patent Application Publication No. 2007-329674).
In addition, in order to execute processing with the communication medium 110 at high speed, the communication apparatus 100 is configured to have a wide communicable range so as to be able to start communication at a distance. This is realized by increasing the transmission power of a transmission amplifier of the communication apparatus 100. By increasing the transmission power of the transmission amplifier, the communication apparatus 100 can perform stable communication with the communication medium 110 located within the communicable range. However, since the communication medium 110 is supplied, by the communication apparatus 100, with the minimum power that is necessary for an internal circuit to operate when the communication medium 110 is separated from the communication apparatus 100 by a maximum communicable distance, the communication medium 110 may be supplied with unnecessarily large power when located close to the communication apparatus 100. Therefore, the communication medium 110 has a protection circuit to prevent the internal circuit from being broken even if excessive power is supplied.